Supermarket data from Kantar Worldpanel revealed that Co-op sales edged 0.2 per cent higher year-on-year in the 12 weeks to July 7, although its market share remained under pressure, dropping from 6.6 per cent to 6.4 per cent.

The group, which is battling to plug a £1.5bn hold in its banking division's balance sheet, has suffered falling food sales since the 12 weeks to February, according to Kantar.

It remains the fifth biggest grocery retailer behind Tesco, Asda, Sainsbury's and Morrisons, but has been losing ground to rivals.

Strong growth from competitors such as Waitrose and the discounters Lidl and Aldi have hit the Co-op, as have increased moves by the big players into the convenience market in recent years.

The Co-op had a combined share of 9.1 per cent at the time of its £1.56bn deal to buy Somerfield in 2008, but has seen this drop sharply since then.

Kantar's figures also showed that Sainsbury's was once more the only one of the "big four" supermarkets not to lose market share in the three months to July 7, holding firm at 16.5 per cent as sales grew 3.8 per cent.

Tesco saw its share slip from 30.7 per cent to 30.1 per cent, while Asda fell from 17.3 per cent to 17 per cent and Morrisons declined from 11.9 per cent to 11.7 per cent.

Waitrose, Aldi and Lidl were the star performers again, with Lidl reaching an all-time high with a 3.1 per cent share and Aldi retaining its record 3.6 per cent.

Sales growth at Waitrose - the supermarket arm of the John Lewis Partnership - stood at 10.9 per cent, nearly three times the market average, giving it a 4.8 per cent share, up from 4.5 per cent a year earlier.

Edward Garner, director at Kantar Worldpanel, said Waitrose, Aldi and Lidl now account for 11.5 per cent of the grocery market, 3.2 percentage points more than they did four years ago.

"This trend has cut deeply into the available market share for the bigger retailers who are now having to compete for a contracting middle ground," he added.